Cold rolling and intercritical annealing of C-Mn steel sheets with different initial microstructures

The microstructure and mechanical properties of cold-rolled dual-phase steels produced from different initial microstructures, i.e., complex ferrite-pearlite, martensite, and bainite, were studied for a C-Mn steel contained 0.15 C, 2.50Mn, and 0.30Si (wt%). Sheets of 4 mm thickness, and with the different initial microstructures were cold rolled for 50% thickness reduction, intercritically annealed at 710 and 740 °C for various times (10-1800 s) in a salt bath, and then water cooled. Different amounts and distributions of martensite were produced from the different initial microstructures by the same annealing treatment. Bainite was the overwhelming initial microstructure, which produced slightly higher amounts of martensite and the net-arrayed martensite distribution. Electron backscattering diffraction revealed recrystallization of cold-rolled bainite before austenite formation at 710 °C; and that the ferrite bound near martensite has remarkable transformation induced effects. Tensile strength and ductility of the steels annealed for 1200 s at 710 °C were almost the same. The effect of the initial microstructure on the tensile properties is remarkable for steels annealed for the same time at 740 °C. The net-arrayed martensite produced by using bainite represented higher initial strain hardening at both temperatures, which is attributed to the transformation effects in the ferrite bound around martensite.